The present invention generally relates to apparatus for heating liquids and, in a preferred embodiment thereof, more particularly relates to apparatus for providing dry fire protection for the resistance type heating elements in electric water heaters.
Water heaters used to heat and store a quantity of water in a tank structure for subsequent on-demand delivery to plumbing fixtures such as sinks, bathtubs and showers in both residences and commercial buildings typically utilize either a combustible fuel such as gas or oil, or one or more electric resistance heating elements, to supply heat to the tank-stored water under the control of a thermostat which monitors the temperature of the stored water. While over the years both of these types of water heaters have evolved into highly reliable heating devices capable of providing years of dependable service, in common with other types of heating equipment they must be installed properly to function as intended.
An electric water heater, like its fuel-fired counterparts, is sold without water in it and is filled with water after it is moved to and installed in its intended operation location. The possibility exists that the water heater can be "dry fired"--i.e., have its electric resistance type heating element(s) energized before the storage tank portion of the heater is filled with water to immerse the heating element(s) projecting into its interior. When such dry firing occurs, each dry fired electric heating element typically burns out, resulting in a return of the unit to the manufacturer, or a service call by a repair technician to perform an on-site element replacement. The cost of either repair procedure can be quite substantial.
Various solutions have previously been proposed to prevent the firing of heating elements in electric water heaters unless the elements are immersed in water introduced into the storage tank portion of the water heater. Primarily, these proposed solutions have taken two forms--float switch-based protective systems, and temperature sensor-based protective systems.
In the float switch system, a buoyant float member is movably supported within the tank and mechanically linked to a normally open electrical switch which, until closed, precludes current flow through the heating element(s). When a quantity of water sufficient to cover all of the electric heating elements is introduced into the tank, the float member is lifted by the water to cause the lifted float to close its associated electrical switch and permit the now immersed heating element to be fired.
In the temperature sensing system, the temperature of the electrical resistance heating element(s) is monitored and, if a preset upper temperature limit (indicative of the dry firing of the element) is sensed, the current flow to the non-immersed element(s) is interrupted to prevent burning out of the element(s).
Neither of these previously proposed dry fire protection techniques has proven to be entirely satisfactory. For example, each tends to be fairly complex and undesirably expensive to incorporate into the overall water heater assembly. Additionally, these previously proposed systems have often proven to be unreliable, and tend to be undesirably invasive of the interior of the storage tank portion of the water heater (particularly in the case of the float switch-based protective system). Moreover, in the case of the float switch-based protective system since movable parts are in contact with the stored water the movable linkage portion of the system can easily become encrusted with scale and be "frozen" over time.
In view of the foregoing it can readily be seen that a need exists for improved apparatus for providing an electric water heater with dry fire protection--particularly during the initial installation of the heater when dry firing of the heating element(s) has proven most likely to occur. It is accordingly an object of the present invention to provide an electric water heater with such improved dry fire protection apparatus.